Method and apparatus to select radio frequency identification devices in accordance with an arbitration scheme

ABSTRACT

A method of establishing wireless communications between an interrogator and individual ones of multiple wireless identification devices, the method comprising utilizing a tree search method to establish communications without collision between the interrogator and individual ones of the multiple wireless identification devices, a search tree being defined for the tree search method, the tree having multiple levels respectively representing subgroups of the multiple wireless identification devices, the method further comprising starting the tree search at a selectable level of the search tree. A communications system comprising an interrogator, and a plurality of wireless identification devices configured to communicate with the interrogator in a wireless fashion, the respective wireless identification devices having a unique identification number, the ingerrogator being configured to employ a tree search technique to determine the unique identification numbers of the different wireless identification devices so as to be able to establish communications between the interrogator and individual ones of the multiple wireless identification devices without collision by multiple wireless identification devices attempting to respond to the interrogator at the same time, wherein the interrogator is configured to start the tree search at a selectable level of the search tree.RFID devices are selected by an interrogator. The interrogator sends a signal to a plurality of RFID devices. The signal indicates a bit string and a memory range comprising multiple bit locations. RFID devices compare the bits stored in their respective memory ranges to the bit string to determine which of the RFID devices are selected.

CROSS REFERENCE TO RELATED APPLICATION APPLICATIONS

ThisMore than one reissue application has been filed for the reissue ofU.S. Pat. No. 6,307,847, including the present reissue application Ser.No. 10/693,696, filed Oct. 23, 2003, a continuation reissue applicationSer. No. 11/859,360, filed Sep. 21, 2007, a continuation reissueapplication Ser. No. 11/859,364, filed Sep. 21, 2007, and a continuationreissue application Ser. No. 12/493,542, filed Jun. 29, 2009. Thepresent application is a reissue application of U.S. Pat. No. 6,307,847,issued from U.S. patent application Ser. No. 09/617,390, filed Jul. 17,2000, and titled “Method of Addressing Messages and CommunicationsSystem,” which is a Continuationcontinuation application of U.S. patentapplication Ser. No. 09/026,043, filed Feb. 19, 1998, and titled “Methodof Addressing Messages and Communications System,” now U.S. Pat. No.6,118,789, each of which is incorporated by reference.

TECHNICAL FIELD

This invention relates to communications protocols and to digital datacommunications. Still more particularly, the invention relates to datacommunications protocols in mediums such as radio communication or thelike. The invention also relates to radio frequency identificationdevices for inventory control, object monitoring, determining theexistence, location or movement of objects, or for remote automatedpayment.

BACKGROUND OF THE INVENTION

Communications protocols are used in various applications. For example,communications protocols can be used in electronic identificationsystems. As large numbers of objects are moved in inventory, productmanufacturing, and merchandising operations, there is a continuouschallenge to accurately monitor the location and flow of objects.Additionally, there is a continuing goal to interrogate the location ofobjects in an inexpensive and streamlined manner. One way of trackingobjects is with an electronic identification system.

One presently available electronic identification system utilizes amagnetic coupling system. In some cases, an identification device may beprovided with a unique identification code in order to distinguishbetween a number of different devices. Typically, the devices areentirely passive (have no power supply), which results in a small andportable package. However, such identification systems are only capableof operation over a relatively short range, limited by the size of amagnetic field used to supply power to the devices and to communicatewith the devices.

Another wireless electronic identification system utilizes a largeactive transponder device affixed to an object to be monitored whichreceives a signal from an interrogator. The device receives the signal,then generates and transmits a responsive signal. The interrogationsignal and the responsive signal are typically radio-frequency (RF)signals produced by an RF transmitter circuit. Because active deviceshave their own power sources, and do not need to be in close proximityto an interrogator or reader to receive power via magnetic coupling.Therefore, active transponder devices tend to be more suitable forapplications requiring tracking of a tagged device that may not be inclose proximity to an interrogator. For example, active transponderdevices tend to be more suitable for inventory control or tracking.

Electronic identification systems can also be used for remote payment.For example, when a radio frequency identification device passes aninterrogator at a toll booth, the toll booth can determine an identityof the radio frequency identification device, and thus of the owner ofthe device, and debit an account held by the owner for payment of tollor can receive a credit card number against which the toll can becharged. Similarly, remote payment is possible for a variety of othergoods or services.

A communication system typically includes two transponders: a commanderstation or interrogator, and a responder station or transponder devicewhich replies to the interrogator.

If the interrogator has prior knowledge of the identification number ofa device which the interrogator is looking for, it can specify that aresponse is requested only from the device with that identificationnumber. Sometimes, such information is not available. For example, thereare occasions where the interrogator is attempting to determine which ofmultiple devices are within communication range.

When the interrogator sends a message to a transponder device requestinga reply, there is a possibility that multiple transponder devices willattempt to respond simultaneously, causing a collision, and thus causingan erroneous message to be received by the interrogator. For example, ifthe interrogator sends out a command requesting that all devices withina communication range identify themselves, and gets a large number ofsimultaneous replies, the interrogator may not be able to interpret anyof these replies. Thus, arbitration schemes are employed to permitcommunications free of collisions.

In one arbitration scheme or system, described in commonly assigned U.S.Pat. Nos. 5,627,544; 5,583,850; 5,500,650; and 5,365,551, all toSnodgrass et al. and all incorporated herein by reference, theinterrogator sends a command causing each device of a potentially largenumber of responding devices to select a random number from a knownrange and use it as that device's arbitration number. By transmittingrequests for identification to various subsets of the full range ofarbitration numbers, and checking for an error-free response, theinterrogator determines the arbitration number of every responderstation capable of communicating at the same time. Therefore, theinterrogator is able to conduct subsequent uninterrupted communicationwith devices, one at a time, by addressing only one device.

Another arbitration scheme is referred to as the Aloha or slotted Alohascheme. This scheme is discussed in various references relating tocommunications, such as Digital Communications: Fundamentals andApplications, Bernard Sklar, published January 1988 by Prentice Hall. Inthis type of scheme, a device will respond to an interrogator using oneof many time domain slots selected randomly by the device. A problemwith the Aloha scheme is that if there are many devices, or potentiallymany devices in the field (i.e. in communications range, capable ofresponding) then there must be many available slots or many collisionswill occur. Having many available slots slows down replies. If themagnitude of the number of devices in a field is unknown, then manyslots are needed. This results in the system slowing down significantlybecause the reply time equals the number of slots multiplied by the timeperiod required for one reply.

An electronic identification system which can be used as a radiofrequency identification device, arbitration schemes, and variousapplications for such devices are described in detail in commonlyassigned U.S. patent application Ser. No. 08/705,043, filed Aug. 29,1996, and now U.S. Pat. No. 6,130,602, which is incorporated herein byreference.

SUMMARY OF THE INVENTION

The invention provides a wireless identification device configured toprovide a signal to identify the device in response to an interrogationsignal.

One aspect of the invention provides a method of establishing wirelesscommunications between an interrogator and individual ones of multiplewireless identification devices. The method comprises utilizing a treesearch method to establish communications without collision between theinterrogator and individual ones of the multiple wireless identificationdevices. A search tree is defined for the tree search method. The treehas multiple levels respectively representing subgroups of the multiplewireless identification devices. The method further comprising startingthe tree search at a selectable level of the search tree. In one aspectof the invention, the method further comprises determining the maximumpossible number of wireless identification devices that couldcommunicate with the interrogator, and selecting a level of the searchtree based on the determined maximum possible number of wirelessidentification devices that could communicate with the interrogator. Inanother aspect of the invention, the method further comprises startingthe tree search at a level determined by taking the base two logarithmof the determined maximum possible number, wherein the level of the treecontaining all subgroups is considered level zero, and lower levels arenumbered consecutively.

Another aspect of the invention provides a communications systemcomprising an interrogator, and a plurality of wireless identificationdevices configured to communicate with the interrogator in a wirelessfashion. The respective wireless identification devices have a uniqueidentification number. The interrogator is configured to employ a treesearch technique to determine the unique identification numbers of thedifferent wireless identification devices so as to be able to establishcommunications between the interrogator and individual ones of themultiple wireless identification devices without collision by multiplewireless identification devices attempting to respond to theinterrogator at the same time. The interrogator is configured to startthe tree search at a selectable level of the search tree.

One aspect of the invention provides a radio frequency identificationdevice comprising an integrated circuit including a receiver, atransmitter, and a microprocessor. In one embodiment, the integratedcircuit is a monolithic single die single metal layer integrated circuitincluding the receiver, the transmitter, and the microprocessor. Thedevice of this embodiment includes an active transponder, instead of atransponder which relies on magnetic coupling for power, and thereforehas a much greater range.

In another aspect, an interrogator sends a signal to a plurality of RFIDdevices. The signal provides a bit string and indicates a memory rangecomprising multiple bit locations. RFID devices compare the bits storedin their respective memory ranges to the bit string to determine whichof the RFID devices are chosen.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below withreference to the following accompanying drawings.

FIG. 1 is a high level circuit schematic showing an interrogator and aradio frequency identification device embodying the invention.

FIG. 2 is a front view of a housing, in the form of a badge or card,supporting the circuit of FIG. 1 according to one embodiment theinvention.

FIG. 3 is a front view of a housing supporting the circuit of FIG. 1according to another embodiment of the invention.

FIG. 4 is a diagram illustrating a tree splitting sort method forestablishing communication with a radio frequency identification devicein a field of a plurality of such devices.

FIG. 5, is a diagram illustrating a modified tree splitting sort methodfor establishing communication with a radio frequency identificationdevice in a field of a plurality of such devices.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstitutional purposes of the U.S. Patent Laws “to promote the progressof science and useful arts” (Article 1, Section 8).

FIG. 1 illustrates a wireless identification device 12 in accordancewith one embodiment of the invention. In the illustrated embodiment, thewireless identification device is a radio frequency data communicationdevice 12, and includes RFID circuitry 16. The device 12 furtherincludes at least one antenna 14 connected to the circuitry 16 forwireless or radio frequency transmission and reception by the circuitry16. In the illustrated embodiment, the RFID circuitry is defined by anintegrated circuit as described in the above-incorporated patentapplication Ser. No. 08/705,043, filed Aug. 29, 1996, now U.S. Pat. No.6,130,602. Other embodiments are possible. A power source or supply 18is connected to the integrated circuit 16 to supply power to theintegrated circuit 16. In one embodiment, the power source 18 comprisesa battery.

The device 12 transmits and receives radio frequency communications toand from an interrogator 26. An exemplary interrogator is described incommonly assigned U.S. patent application Ser. No. 08/907,689, filedAug. 8, 1997and , now U.S. Pat. No. 6,289,209, which is incorporatedherein by reference. Preferably, the interrogator 26 includes an antenna28, as well as dedicated transmitting (e.g., modulator) and receivingcircuitry, similar to that implemented on the integrated circuit 16.

Generally, the interrogator 26 transmits an interrogation signal orcommand 27 via the antenna 28. The device 12 receives the incominginterrogation signal via its antenna 14. Upon receiving the signal 27,the device 12 responds by generating and transmitting a responsivesignal or reply 29. The responsive signal 29 typically includesinformation that uniquely identifies, or labels the particular device 12that is transmitting, so as to identify any object or person with whichthe device 12 is associated.

Although only one device 12 is shown in FIG. 1, typically there will bemultiple devices 12 that correspond with the interrogator 26, and theparticular devices 12 that are in communication with the interrogator 26will typically change over time. In the illustrated embodiment in FIG.1, there is no communication between multiple devices 12. Instead, thedevices 12 respectively communicate with the interrogator 26. Multipledevices 12 can be used in the same field of an interrogator 26 (i.e.,within communications range of an interrogator 26).

The radio frequency data communication device 12 can be included in anyappropriate housing or packaging. Various methods of manufacturinghousings are described in commonly assigned U.S. patent application Ser.No. 08/800,037, filed Feb. 13, 1997, and now U.S. Pat. No. 5,988,510,which is incorporated herein by reference.

FIG. 2 shows but one embodiment in the form of a card or badge 19including a housing 11 of plastic or other suitable material supportingthe device 12 and the power supply 18. In one embodiment, the front faceof the badge has visual identification features such as graphics, text,information found on identification or credit cards, etc.

FIG. 3 illustrates but one alternative housing supporting the device 12.More particularly, FIG. 3 shows a miniature housing 20 encasing thedevice 12 and power supply 18 to define a tag which can be supported byan object (e.g., hung from an object, affixed to an object, etc.).Although two particular types of housings have been disclosed, thedevice 12 can be included in any appropriate housing.

If the power supply 18 is a battery, the battery can take any suitableform. Preferably, the battery type will be selected depending on weight,size, and life requirements for a particular application. In oneembodiment, the battery 18 is a thin profile button-type cell forming asmall, thin energy cell more commonly utilized in watches and smallelectronic devices requiring a thin profile. A conventional button-typecell has a pair of electrodes, an anode formed by one face and a cathodeformed by an opposite face. In an alternative embodiment, the powersource 18 comprises a series connected pair of button type cells.Instead of using a battery, any suitable power source can be employed.

The circuitry 16 further includes a backscatter transmitter and isconfigured to provide a responsive signal to the interrogator 26 byradio frequency. More particularly, the circuitry 16 includes atransmitter, a receiver, and memory such as is described in U.S. patentapplication Ser. No. 08/705,043, now U.S. Pat. No. 6,130,602.

Radio frequency identification has emerged as a viable and affordablealternative to tagging or labeling small to large quantities of items.The interrogator 26 communicates with the devices 12 via anelectromagnetic link, such as via an RF link (e.g., at microwavefrequencies, in one embodiment), so all transmissions by theinterrogator 26 are heard simultaneously by all devices 12 within range.

If the interrogator 26 sends out a command requesting that all devices12 within range identify themselves, and gets a large number ofsimultaneous replies, the interrogator 26 may not be able to interpretany of these replies. Therefore, arbitration schemes are provided.

If the interrogator 26 has prior knowledge of the identification numberof a device 12 which the interrogator 26 is looking for, it can specifythat a response is requested only from the device 12 with thatidentification number. To target a command at a specific device 12,(i.e., to initiate point-on-point communication), the interrogator 26must send a number, identifying a specific device 12 along with thecommand. At start-up, or in a new or changing environment, theseidentification numbers are not known by the interrogator 26. Therefore,the interrogator 26 must identify all devices 12 in the field (withincommunication range) such as by determining the identification numbersof the devices 12 in the field. After this is accomplished,point-to-point communication can proceed as desired by the interrogator26.

Generally speaking, RFID systems are a type of multi-accesscommunication system. The distance between the interrogator 26 anddevices 12 within the field is typically fairly short (e.g., severalmeters), so packet transmission time is determined primarily by packetsize and baud rate. Propagation delays are negligible. In such systems,there is a potential for a large number of transmitting devices 12 andthere is a need for the interrogator 26 to work in a changingenvironment, where different devices 12 are swapped in and outfrequently (e.g., as inventory is added or removed). In such systems,the inventors have determined that the use of random access methods workeffectively for contention resolution (i.e., for dealing with collisionsbetween devices 12 attempting to respond to the interrogator 26 at thesame time).

RFID systems have some characteristics that are different from othercommunications systems. For example, one characteristic of theillustrated RFID systems is that the devices 12 never communicatewithout being prompted by the interrogator 26. This is in contrast totypical multiaccess systems where the transmitting units operate moreindependently. In addition, contention for the communication medium isshort lived as compared to the ongoing nature of the problem in othermultiaccess systems. For example, in a RFID system, after the devices 12have been identified, the interrogator can communicate with them in apoint-to-point fashion. Thus, arbitration in a RFID system is atransient rather than steady-state phenomenon. Further, the capabilityof a device 12 is limited by practical restrictions on size, power, andcost. The lifetime of a device 12 can often be measured in terms ofnumber of transmissions before battery power is lost. Therefore, one ofthe most important measures of system performance in RFID arbitration istotal time required to arbitrate a set of devices 12. Another measure ispower consumed by the devices 12 during the process. This is in contrastto the measures of throughput and packet delay in other types ofmultiaccess systems.

FIG. 4 illustrates one arbitration scheme that can be employed forcommunication between the interrogator and devices 12. Generally, theinterrogator 26 sends a command causing each device 12 of a potentiallylarge number of responding devices 12 to select a random number from aknown range and use it as that device's arbitration number. Bytransmitting requests for identification to various subsets of the fullrange of arbitration numbers, and checking for an error-free response,the interrogator 26 determines the arbitration number of every responderstation capable of communicating at the same time. Therefore, theinterrogator 26 is able to conduct subsequent uninterruptedcommunication with devices 12, one at a time, by addressing only onedevice 12.

Three variables are used: an arbitration value (AVALUE), an arbitrationmask (AMASK), and a random value ID (RV). The interrogator sends anIdentify command (IdentifyCmnd) causing each device of a potentiallylarge number of responding devices to select a random number from aknown range and use it as that device's arbitration number. Theinterrogator sends an arbitration value (AVALUE) and an arbitration mask(AMASK) to a set of devices 12. The receiving devices 12 evaluate thefollowing equation: (AMASK & AVALUE)==(AMASK & RV) wherein “&” is abitwise AND function, and wherein “═” is an equality function. If theequation evaluates to “1” (TRUE), then the device 12 will reply. If theequation evaluates to “0” (FALSE), then the device 12 will not reply. Byperforming this in a structured manner, with the number of bits in thearbitration mask being increased by one each time, eventually a device12 will respond with no collisions. Thus, a binary search treemethodology is employed.

An example using actual numbers will now be provided using only fourbits, for simplicity, reference being made to FIG. 4. In one embodiment,sixteen bits are used for AVALUE and AMASK. Other numbers of bits canalso be employed depending, for example, on the number of devices 12expected to be encountered in a particular application, on desired costpoints, etc.

Assume, for this example, that there are two devices 12 in the field,one with a random value (RV) of 1100 (binary), and another with a randomvalue (RV) of 1010 (binary). The interrogator is trying to establishcommunications without collisions being caused by the two devices 12attempting to communicate at the same time.

The interrogator sets AVALUE to 0000 (or “don't care” for all bits, asindicated by the character “X” in FIG. 4) and AMASK to 0000. Theinterrogator transmits a command to all devices 12 requesting that theyidentify themselves. Each of the devices 12 evaluate (AMASK &AVALUE)═(AMASK & RV) using the random value RV that the respectivedevices 12 selected. If the equation evaluates to “1” (TRUE), then thedevice 12 will reply. If the equation evaluates to “0” (FALSE), then thedevice 12 will not reply. In the first level of the illustrated tree,AMASK is 0000 and anything bitwise ANDed with all zeros results in allzeros, so both the devices 12 in the field respond, and there is acollision.

Next, the interrogator sets AMASK to 0001 and AVALUE to 0000 andtransmits an identify command. Both devices 12 in the field have a zerofor their least significant bit, and (AMASK & AVALUE)═(AMASK & RV) willbe true for both devices 12. For the device 12 with a random value of1100, the left side of the equation is evaluated as follows (0001 &0000)=0000. The right side is evaluated as (0001 & 1100)=0000. The leftside equals the right side, so the equation is true for the device 12with the random value of 1100. For the device 12 with a random value of1010, the left side of the equation is evaluated as (0001 & 0000)=0000.The right side is evaluated as (0001 & 1010)=0000. The left side equalsthe right side, so the equation is true for the device 12 with therandom value of 1010. Because the equation is true for both devices 12in the field, both devices 12 in the field respond, and there is anothercollision.

Recursively, the interrogator next sets AMASK to 0011 with AVALUE stillat 0000 and transmits an Identify command. (AMASK & AVALUE)═(AMASK & RV)is evaluated for both devices 12. For the device 12 with a random valueof 1100, the left side of the equation is evaluated as follows (0011 &0000)=0000. The right side is evaluated as (0011 & 1100)=0000. The leftside equals the right side, so the equation is true for the device 12with the random value of 1100, so this device 12 responds. For thedevice 12 with a random value of 1010, the left side of the equation isevaluated as (0011 & 0000)=0000. The right side is evaluated as (0011 &1010)=0010. The left side does not equal the right side, so the equationis false for the device 12 with the random value of 1010, and thisdevice 12 does not respond. Therefore, there is no collision, and theinterrogator can determine the identity (e.g., an identification number)for the device 12 that does respond.

De-recursion takes place, and the devices 12 to the right for the sameAMASK level are accessed when AVALUE is set at 0010, and AMASK is set to0011.

The device 12 with the random value of 1010 receives a command andevaluates the equation (AMASK & AVALUE)═(AMASK & RV). The left side ofthe equation is evaluated as (0011 & 0010)=0010. The right side of theequation is evaluated as (0011 & 1010)=0010. The right side equals theleft side, so the equation is true for the device 12 with the randomvalue of 1010. Because there are no other devices 12 in the subtree, agood reply is returned by the device 12 with the random value of 1010.There is no collision, and the interrogator 26 can determine theidentity (e.g., an identification number) for the device 12 that doesrespond.

By recursion, what is meant is that a function makes a call to itself.In other words, the function calls itself within the body of thefunction. After the called function returns, de-recursion takes placeand execution continues at the place just after the function call: i.e.at the beginning of the statement after the function call.

For instance, consider a function that has four statements (numbered1,2,3,4 ) in it, and the second statement is a recursive call. Assumethat the fourth statement is a return statement. The first time throughthe loop (iteration 1) the function executes the statement 2 and(because it is a recursive call) calls itself causing iteration 2 tooccur. When iteration 2 gets to statement 2, it calls itself makingiteration 3. During execution in iteration 3 of statement 1, assume thatthe function does a return. The information that was saved on the stackfrom iteration 2 is loaded and the function resumes execution atstatement 3 (in iteration 2), followed by the execution of statement 4which is also a return statement. Since there are no more statements inthe function, the function de-recurses to iteration 1. Iteration 1, hadpreviously recursively called itself in statement 2. Therefore, it nowexecutes statement 3 (in iteration 1 ). Following that it executes areturn at statement 4. Recursion is known in the art.

Consider the following code which can be used to implement operation ofthe method shown in FIG. 4 and described above.

Arbitrate(AMASK, AVALUE) { collision=IdentifyCmnd(AMASK,AVALUE) if(collision) then { /* recursive call for left side */Arbitrate((AMASK>>1)+1, AVALUE) /* recursive call for right side */Arbitrate((AMASK>>1)+1, AVALUE+(AMASK+1)) } /* endif */ }/* return */

The symbol “<<” represents a bitwise left shift. “<<” means shift leftby one place. Thus, 0001<<1 would be 0010. Note, however, that AMASK isoriginally called with a value of zero, and 0000<<1 is still 0000.Therefore, for the first recursive call, AMASK=(AMASK<<1)+1. So for thefirst recursive call, the vale of AMASK is 0000+0001=0001. For thesecond call, AMASK=(0001<<)+1=0010+1=0011. For the third recursive call,AMASK=(0011<<1)+1=0110+1=0111.

The routine generates values for AMASK and AVALUE to be used by theinterrogator in an identify command “IdentifyCmnd.” Note that theroutine calls itself it there is a collision. De-recursion occurs whenthere is no collision. AVALUE and AMASK would have values such as thefollowing assuming collisions take place all the way down to the bottomof the tree.

AVALUE AMASK 0000 0000 0000 0001 0000 0011 0000 0111 0000  1111* 1000 1111* 0100 0111 0100  1111* 1100  1111*

This sequence of AMASK, AVALUE binary numbers assumes that there arecollisions all the way down to the bottom of the tree, at which pointthe Identify command sent by the interrogator is finally successful sothat no collision occurs. Rows in the table for which the interrogatoris successful in receiving a reply without collision are marked with thesymbol “*”. Note that if the Identify command was successful at, forexample, the third line in the table then the interrogator would stopgoing down that branch of the tree and start down another, so thesequence would be as shown in the following table.

AVALUE AMASK 0000 0000 0000 0001 0000  0011* 0010 0011 . . . . . .

This method is referred to as a splitting method. It works by splittinggroups of colliding devices 12 into subsets that are resolved in turn.The splitting method can also be viewed as a type of tree search. Eachsplit moves the method one level deeper in the tree.

Either depth-first or breadth-first traversals of the tree can beemployed Depth first traversals are performed by using recursion, as isemployed in the code listed above. Breadth-first traversals areaccomplished by using a queue instead of recursion. The following is anexample of code for performing a breadth-first traversal.

Arbitrate(AMASK, AVALUE) { enqueue(0,0) while (queue != empty) (AMASK,AVALUE) = dequeue( ) collision=IdentifyCmnd(AMASK,AVALUE) if (collision)then { TEMP = AMASK+1 NEW_AMASK = (AMASK>>1)+1 enqueue(NEW_AMASK,AVALUE) enqueue(NEW_AMASK, AVALUE+TEMP) } /* endif */ endwhile }/*return */

The symbol “!=” means not equal to. AVALUE and AMASK would have valuessuch as those indicated in the following table for such code.

AVALUE AMASK 0000 0000 0000 0001 0001 0001 0000 0011 0010 0011 0001 00110011 0011 0000 0111 0100 0111 . . . . . .

Rows in the table for which the interrogator is successful in receivinga reply without collision are marked with the symbol “*”.

FIG. 5 illustrates an embodiment wherein the interrogator 26 starts thetree search at a selectable level of the search tree. The search treehas a plurality of nodes 51, 52, 53, 54 etc. at respective levels. Thesize of subgroups of random values decrease in size by half with eachnode descended. The upper bound of the number of devices 12 in the field(the maximum possible number of devices that could communicate with theinterrogator) is determined, and the tree search method is started at alevel 32, 34, 36, 38, or 40 in the tree depending on the determinedupper bound. In one embodiment, the maximum number of devices 12potentially capable of responding to the interrogator is determinedmanually and input into the interrogator 26 via an input device such asa keyboard, graphical user interface, mouse, or other interface. Thelevel of the search tree on which to start the tree search is selectedbased on the determined maximum possible number of wirelessidentification devices that could communicate with the interrogator.

The tree search is started at a level determined by taking the base twologarithm of the determined maximum possible number. More particularly,the tree search is started at a level determined by taking the base twologarithm of the power of two nearest the determined maximum possiblenumber of devices 12. The level of the tree containing all subgroups ofrandom values is considered level zero (see FIG. 5), and lower levelsare numbered 1, 2, 3, 4, etc. consecutively.

By determining the upper bound of the number of devices 12 in the field,and starting the tree search at an appropriate level, the number ofcollisions is reduced, the battery life of the devices 12 is increased,and arbitration time is reduced.

For example, for the search tree shown in FIG. 5, if it is known thatthere are seven devices 12 in the field, starting at node 51 (level 0 )results in a collision. Starting at level 1 (nodes 52 and 53 ) alsoresults in a collision. The same is true for nodes 54, 55, 56, and 57 inlevel 2. If there are seven devices 12 in the field, the nearest powerof two to seven is the level at which the tree search should be started.Log₂ 8=3, so the tree search should be started at level 3 if there areseven devices 12 in the field.

AVALUE and AMASK would have values such as the following assumingcollisions take place from level 3 all the way down to the bottom of thetree.

AVALUE AMASK 0000 0111  0000 1111* 1000 1111* 0100 0111  0100 1111* 11001111*

Rows in the table for which the interrogator is successful in receivinga reply without collision are marked with the symbol “*”.

In operation, the interrogator transmits a command requesting devices 12having random values RV within a specified group of random values torespond, the specified group being chosen in response to the determinedmaximum number. Devices 12 receiving the command respectively determineif their chosen random values fall within the specified group and, ifso, send a reply to the interrogator. The interrogator determines if acollision occurred between devices that sent a reply and, if so, createsa new, smaller, specified group, descending in the tree, as describedabove in connection with FIG. 4.

Another arbitration method that can be employed is referred to as the“Aloha” method. In the Aloha method, every time a device 12 is involvedin a collision, it waits a random period of time before retransmitting.This method can be improved by dividing time into equally sized slotsand forcing transmissions to be aligned with one of these slots. This isreferred to as “slotted Aloha.” In operation, the interrogator asks alldevices 12 in the field to transmit their identification numbers in thenext time slot. If the response is garbled, the interrogator informs thedevices 12 that a collision has occurred, and the slotted Aloha schemeis put into action. This means that each device 12 in the field respondswithin an arbitrary slot determined by a randomly selected value. Inother words, in each successive time slot, the devices 12 decide totransmit their identification number with a certain probability.

The Aloha method is based on a system operated by the University ofHawaii. In 1971, the University of Hawaii began operation of a systemnamed Aloha. A communication satellite was used to interconnect severaluniversity computers by use of a random access protocol. The systemoperates as follows. Users or devices transmit at any time they desire.After transmitting, a user listens for an acknowledgment from thereceiver or interrogator. Transmissions from different users willsometimes overlap in time (collide), causing reception errors in thedata in each of the contending messages. The errors are detected by thereceiver, and the receiver sends a negative acknowledgment to the users.When a negative acknowledgment is received, the messages areretransmitted by the colliding users after a random delay. If thecolliding users attempted to retransmit without the random delay, theywould collide again. If the user does not receive either anacknowledgment or a negative acknowledgment within a certain amount oftime, the user “times out” and retransmits the message.

There is a scheme known as slotted Aloha which improves the Aloha schemeby requiring a small amount of coordination among stations. In theslotted Aloha scheme, a sequence of coordination pulses is broadcast toall stations (devices). As is the case with the pure Aloha scheme,packet lengths are constant. Messages are required to be sent in a slottime between synchronization pulses, and can be started only at thebeginning of a time slot. This reduces the rate of collisions becauseonly messages transmitted in the same slot can interfere with oneanother. The retransmission mode of the pure Aloha scheme is modifiedfor slotted Aloha such that if a negative acknowledgment occurs, thedevice retransmits after a random delay of an integer number of slottimes.

Aloha methods are described in a commonly assigned patent applicationnaming Clifton W. Wood, Jr. as an inventor, U.S. patent application Ser.No. 09/026,248, filed Feb. 19, 1998, titled “Method of AddressingMessages and Communications System,” filed concurrently herewith, and ,now U.S. Pat. No. 6,275,476, which is incorporated herein by reference.

In one alternative embodiment, an Aloha method (such as the methoddescribed in the commonly assigned patent application mentioned above)is combined with determining the upper bound on a set of devices andstarting at a level in the tree depending on the determined upper bound,such as by combining an Aloha method with the method shown and describedin connection with FIG. 5. For example, in one embodiment, devices 12sending a reply to the interrogator 26 do so within a randomly selectedtime slot of a number of slots.

In another embodiment, levels of the search tree are skipped. Skippinglevels in the tree, after a collision caused by multiple devices 12responding, reduces the number of subsequent collisions without addingsignificantly to the number of no replies. In real-time systems, it isdesirable to have quick arbitration sessions on a set of devices 12whose unique identification numbers are unknown. Level skipping reducesthe number of collisions, both reducing arbitration time and conservingbattery life on a set of devices 12. In one embodiment, every otherlevel is skipped. In alternative embodiments, more than one level isskipped each time.

The trade off that must be considered in determining how many (if any)levels to skip with each decent down the tree is as follows. Skippinglevels reduces the number of collisions, thus saving battery power inthe devices 12. Skipping deeper (skipping more than one level) furtherreduces the number of collisions. The more levels that are skipped, thegreater the reduction in collisions. However, skipping levels results inlonger search times because the number of queries (Identify commands)increases. The more levels that are skipped, the longer the searchtimes. Skipping just one level has an almost negligible effect on searchtime, but drastically reduces the number of collisions. If more than onelevel is skipped, search time increases substantially. Skipping everyother level drastically reduces the number of collisions and savesbattery power without significantly increasing the number of queries.

Level skipping methods are described in a commonly assigned patentapplication 09/026,045 naming Clifton W. Wood, Jr. and Don Hush asinventors, titled “Method of Addressing Messages, Method of EstablishingWireless Communications, and Communications Systems,” filed concurrentlyherewith, now U.S. Pat. No. 6,072,801, and incorporated herein byreference.

In one alternative embodiment, a level skipping method is combined withdetermining the upper bound on a set of devices and starting at a levelin the tree depending on the determined upper bound, such as bycombining a level skipping method with the method shown and described inconnection with FIG. 5.

In yet another alternative embodiment, both a level skipping method andan Aloha method (as described in the commonly assigned applicationsdescribed above) are combined with the method shown and described inconnection with FIG. 5.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

1. A method of establishing wireless communications between aninterrogator and individual ones of multiple wireless identificationdevices, the wireless identification devices having respectiveidentification numbers and being addressable by specifyingidentification numbers with any one of multiple possible degrees ofprecision, the method comprising utilizing a tree search in anarbitration scheme to determine a degree of precision necessary toestablish one-to-one communications between the interrogator andindividual ones of the multiple wireless identification devices, asearch tree being defined for the tree search method, the tree havingmultiple selectable levels respectively representing subgroups of themultiple wireless identification devices, the level at which a treesearch starts being variable the method further comprising starting thetree search at any selectable level of the search tree selectable levelother than the top level of the search tree.
 2. A method in accordancewith claim 1 and further comprising determining the maximum possiblenumber of wireless identification devices that could communicate withthe interrogator, and selecting a level of the search tree based of thedetermined maximum possible number of wireless identification devicesthat could communicate with the interrogator.
 3. A method in accordancewith claim 2 and further comprising starting the tree search at a leveldetermined by taking the base two logarithm of the determined maximumpossible number, wherein the level of the tree containing all subgroupsis considered level zero, and lower levels are numbered consecutively.4. A method in accordance with claim 2 and further comprising startingthe tree search at a level determined by taking the base two logarithmof the determined maximum possible number, wherein the level of the treecontaining all subgroups is considered level zero, and lower levels arenumbered consecutively, and wherein the maximum number of devices in asubgroup in one level is half of the maximum number of devices in thenext higher level.
 5. A method in accordance with claim 2 and furthercomprising starting the tree search at a level determined by taking thebase two logarithm of the power of two nearest the determined maximumpossible number, wherein the level of the tree containing all subgroupsis considered level zero, and lower levels are numbered consecutively,and wherein the maximum number of devices in a subgroup in one level ishalf of the maximum number of devices in the next higher level.
 6. Amethod in accordance with claim 1 wherein the wireless identificationdevice comprises an integrated circuit including a receiver, amodulator, and a microprocessor in communication with the receiver andmodulator.
 7. A method of addressing messages from an interrogator to aselected one or more of a number of communications device, the methodcomprising: establishing for respective devices unique identificationnumbers respectively having a first predetermined number of bits;establishing a second predetermined number of bits to be used for randomvalues; causing the devices to select random values, wherein respectivedevices choose random values independently of random values selected bythe other devices; determining the maximum number of devices potentiallycapable of responding to the interrogator; transmitting a command fromthe interrogator requesting devices having random values within aspecified group of random values to respond, by using a subset of thesecond predetermined number of bits, the specified group being chosen inresponse to the determined maximum number; receiving the command atmultiple devices, devices receiving the command respectively determiningif the random value chosen by the device falls within the specifiedgroup and, if so, sending a reply to the interrogator; and determiningusing the interrogator if a collision occurred between devices that senta reply and, if so, creating a new, smaller, specified group.
 8. Amethod of addressing messages from an interrogator to a selected one ormore of a number of communications devices in accordance with claim 7wherein sending a reply to the interrogator comprises transmitting theunique identification number of the device sending the reply.
 9. Amethod of addressing messages from an interrogator to a selected one ormore of a number of communications devices in accordance with claim 7wherein sending a reply to the interrogator comprises transmitting therandom value of the device sending the reply.
 10. A method of addressingmessages from an interrogator to a selected one or more of a number ofcommunications devices in accordance with claim 7 wherein sending areply to the interrogator comprises transmitting both the random valueof the device sending the reply and the unique identification number ofthe device sending the reply.
 11. A method of addressing messages froman interrogator to a selected one or more of a number of communicationsdevices in accordance with claim 7 wherein, after receiving a replywithout collision from a device, the interrogator sends a commandindividually addressed to that device.
 12. A method of addressingmessages from an interrogator to a selected one or more of a number ofcommunications devices, the method comprising: causing the devices toselect random values for use as arbitration numbers, wherein respectivedevices choose random values independently of random values selected bythe other devices, the devices being addressable by specifyingarbitration numbers with any one of multiple possible degrees ofprecision; transmitting a command from the interrogator requestingdevices having random values within a specified group of a plurality ofpossible groups of random values to respond, the specified group beingless than the entire set of random values, the plurality of possiblegroups being organized in a binary tree defined by a plurality of nodesat respective levels, wherein the size of groups of random valuesdecrease in size by half with each node descended, wherein the specifiedgroup is below a node on the tree selected based on the maximum numberof devices capable of communicating with the interrogator; receiving thecommand at multiple devices, devices receiving the command respectivelydetermining if the random value chosen by the device falls within thespecified group and, if so, sending a reply to the interrogator; and, ifnot, not sending a reply; and determining using the interrogator if acollision occurred between devices that sent a reply and, if so,creating a new, smaller, specified group by descending in the tree. 13.A method of addressing messages from an interrogator to a selected oneor more of a number of communications devices in accordance with claim12 and further including establishing a predetermined number of bits tobe used for the random values.
 14. A method of addressing messages froman interrogator to a selected one or more of a number of communicationsdevices in accordance with claim 13 wherein the predetermined number ofbits to be used for the random values comprises an integer multiple ofeight.
 15. A method of addressing messages from an interrogator to aselected one or more of a number of communications devices in accordancewith claim 13 wherein devices sending a reply to the interrogator do sowithin a randomly selected time slot of a number of slots.
 16. A methodof addressing messages from an interrogator to a selected one or more ofa number of RFID devices, the method comprising: establishing forrespective devices a predetermined number of bits to be used for randomvalues, the predetermined number being a multiple of sixteen; causingthe devices to select random values, wherein respective devices chooserandom values independently of random values selected by the otherdevices; transmitting a command from the interrogator requesting deviceshaving random values within a specified group of a plurality of possiblegroups of random values to respond, the specified group being equal toor less than the entire set of random values, the plurality of possiblegroups being organized in a binary tree defined by a plurality of nodesat respective levels, wherein the maximum size of groups of randomvalues decrease in size by half with each node descended, wherein thespecified group is below a node on a level of the tree selected based onthe maximum number of devices known to be capable of communicating withthe interrogator; receiving the command at multiple devices, devicesreceiving the command respectively determining if the random valuechosen by the device falls within the specified group and, only if so,sending a reply to the interrogator, wherein sending a reply to theinterrogator comprises transmitting both the random value of the devicesending the reply and the unique a unique identification number of thedevice sending the reply; using the interrogator to determine if acollision occurred between devices that sent a reply and, if so,creating a new, smaller, specified group using a level of the treedifferent from the level used in the interrogator transmitting, theinterrogator transmitting a command requesting devices having randomvalues within the new specified group of random values to respond; andif a reply without collision is received from a device, the interrogatorsubsequently sending a command individually addressed to that device.17. A method of addressing messages from an interrogator to a selectedone or more of a number of RFID devices in accordance with claim 16 andfurther comprising determining the maximum possible number of wirelessidentification devices that could communicate with the interrogator. 18.A method of addressing messages from an interrogator to a selected oneor more of a number of RFID devices in accordance with claim 16 claim 17wherein selecting the level of the tree comprises taking the base twologarithm of the determined maximum possible number, wherein a level ofthe tree containing all subgroups is considered level zero, and lowerlevels are numbered consecutively.
 19. A method of addressing messagesfrom an interrogator to a selected one or more of a number of RFIDdevices in accordance with claim 16 claim 17 wherein selecting the levelof the tree comprises taking the base two logarithm of the determinedmaximum possible number, wherein a level of the tree containing allsubgroups is considered level zero, and lower levels are numberedconsecutively, and wherein the maximum number of devices in a subgroupin one level is half of the maximum number of devices in the next higherlevel.
 20. A method of addressing messages from an interrogator to aselected one or more of a number of RFID devices in accordance withclaim 16 claim 17 wherein selecting the level of the tree comprisestaking the base two logarithm of the power of two nearest the determinedmaximum possible number, wherein the level of the tree containing allsubgroups is considered level zero, and lower levels are numberedconsecutively, and wherein the maximum number of devices in a subgroupin one level is half of the maximum number of devices in the next higherlevel.
 21. A method of addressing messages from an interrogator to aselected one or more of a number of RFID devices in accordance withclaim 16 wherein the wireless identification device comprises anintegrated circuit including a receiver, a modulator, and amicroprocessor in communication with the receiver and modulator.
 22. Amethod of addressing messages from an interrogator to a selected one ormore of a number of RFID devices in accordance with claim 16 and furthercomprising, after the interrogator transmits a command requestingdevices having random values within the new specified group of randomvalues to respond, determining, using devices receiving the command, iftheir chosen random values fall within the new smaller specified groupand, if so, sending a reply to the interrogator.
 23. A method ofaddressing messages from an interrogator to a selected one or more of anumber of RFID devices in accordance with claim 22 and furthercomprising, after the interrogator transmits a command requestingdevices having random values within the new specified group of randomvalues to respond, determining if a collision occurred between devicesthat sent a reply and, if so, creating a new specified group andrepeating the transmitting of the command requesting devices havingrandom values within a specified group of random values to respond usingdifferent specified groups until all of the devices withincommunications range are identified.
 24. A communications systemcomprising an interrogator, and a plurality of wireless identificationdevices configured to communicate with the interrogator in a wirelessfashion, the wireless identification devices having respectiveidentification numbers, the interrogator being configured to employ atree search in a search tree having multiple selectable levels, todetermine the identification numbers of the different wirelessidentification devices with sufficient precision so as to be able toestablish one-on-one communications between the interrogator andindividual ones of the multiple wireless identification devices, whereinthe interrogator is configured to start the tree search at anyselectable level of the search tree selectable level other than the toplevel of the search tree.
 25. A communications system in accordance withclaim 24 wherein the tree search is a binary tree search.
 26. Acommunications system in accordance with claim 24 wherein the wirelessidentification device comprises an integrated circuit including areceiver, a modulator, and a microprocessor in communication with thereceiver and modulator.
 27. A system comprising: an interrogator; anumber of communications devices capable of wireless communications withthe interrogator; means for establishing a predetermined number of bitsto be used as random numbers, and for causing respective devices toselect random numbers respectively having the predetermined number ofbits; means for inputting a predetermined number indicative of themaximum number of devices possibly capable of communicating with thereceiver interrogator; means for causing the interrogator to transmit acommand requesting devices having random values within a specified groupof random values to respond, the specified group being chosen inresponse to the inputted predetermined number; means for causing devicesreceiving the command to determine if their chosen random values fallwithin the specified group and, if so, send a reply to the interrogator;and means for causing the interrogator to determine if a collisionoccurred between devices that sent a reply and, if so, create a new,smaller, specified group.
 28. A system in accordance with claim 27wherein sending a reply to the interrogator comprises transmitting therandom value of the device sending the reply.
 29. A system in accordancewith claim 27 wherein the interrogator further includes means for, afterreceiving a reply without collision from a device, sending a commandindividually addressed to that device.
 30. A system comprising: aninterrogator configured to communicate to a selected one or more of anumber of communications devices; a plurality of communications devices;the devices being configured to select random values, wherein respectivedevices choose random values independently of random values selected bythe other devices, different sized groups of devices being addressableby specifying random values with differing levels of precision; theinterrogator being configured to transmit a command requesting deviceshaving random values within a specified group of a plurality of possiblegroups of random values to respond, the specified group being less thanthe entire set of random values, the plurality of possible groups beingorganized in a binary tree defined by a plurality of nodes at respectivelevels, wherein the size of groups of random values decrease in size byhalf with each node descended, wherein the specified group is below anode on the tree selected based on a predetermined maximum number ofdevices capable of communicating with the interrogator; devicesreceiving the command being configured to respectively determine iftheir chosen random values fall within the specified group and, if so,send a reply to the interrogator; and, if not, not send a reply; and theinterrogator being configured to determine if a collision occurredbetween devices that sent a reply and, if so, create a new, smaller,specified group by descending in the tree.
 31. A system in accordancewith claim 30 wherein the random values respectively have apredetermined number of bits.
 32. A system in accordance with claim 30wherein respective devices are configured to store unique identificationnumbers of a predetermined number of bits.
 33. A system in accordancewith claim 30 wherein respective devices are configured to store uniqueidentification numbers of sixteen bits.
 34. A system comprising: aninterrogator configured to communicate to a selected one or more of anumber of RFID devices; a plurality of RFID devices, respective devicesbeing configured to store unique identification numbers respectivelyhaving a first predetermined number of bits, respective devices beingfurther configured to store a second predetermined number of bits to beused for random values, respective devices being configured to selectrandom values independently of random values selected by the otherdevices; the interrogator being configured to transmit an identifycommand requesting a response from devices having random values within aspecified group of a plurality of possible groups or random of randomvalues, the specified group being less than or equal to the entire setof random values, the plurality of possible groups being organized in abinary tree defined by a plurality of nodes at respective levels,wherein the maximum size of groups of random values decrease in size byhalf with each node descended, wherein the specified group is below anode on a level of the tree selected based on the maximum number ofdevices known to be capable of communicating with the interrogator;devices receiving the command respectively being configured to determineif their chosen random values fall within the specified group and, onlyif so, send a reply to the interrogator, wherein sending a reply to theinterrogator comprises transmitting both the random value of the devicesending the reply and the unique identification number of the devicesending the reply; the interrogator being configured to determine if acollision occurred between devices that sent a reply and, if so, createa new, smaller, specified group using a level of the tree different fromthe level used in previously transmitting an identify command, theinterrogator transmitting an identify command requesting devices havingrandom values within the new specified group of random values torespond; and the interrogator being configured to send a commandindividually addressed to a device after communicating with a devicewithout a collision.
 35. A system in accordance with 34 wherein theinterrogator is configured to input and store the predetermined number anumber representing the specified group.
 36. A system in accordance with34 wherein the devices are configured to respectively determine if theirchosen random values fall within a specified group and, if so, send areply, upon receiving respective identify commands.
 37. A system inaccordance with claim 36 wherein the interrogator is configured todetermine if a collision occurred between devices that sent a reply inresponse to respective identify commands and, if so, create further newspecified groups and repeat the transmitting of the identify commandrequesting devices having random values within a specified group ofrandom values to respond using different specified groups until allresponding devices are identified.
 38. A method comprising: disposing aplurality of radio frequency identification (RFID) tags in acommunication field of an interrogator, each respective tag of theplurality of tags including respective memory storing a respectiveidentification code that identifies a respective object to which eachrespective tag is affixed; sending a select command from theinterrogator to the plurality of tags after disposing the plurality oftags in the field and before any of the plurality of tags communicate tothe interrogator, the select command including a set of parameters, theset of parameters including a bit string and describing a memory range,the memory range comprising multiple bit locations; each respective tagof the plurality of tags receiving the select command and comparing thebit string against the memory range of the respective memory of eachrespective tag to determine if the respective tag is a member of apopulation of tags; each respective tag of the population picking arespective random value and associating the random value with arespective slot, wherein a sequence in which the population of tags areto reply to the interrogator is determined by each respective slot; eachrespective tag of at least a portion of the population backscattering arespective reply to the interrogator, each respective reply including arespective random number generated by each respective tag, eachrespective tag replying in accordance with the sequence; and sending anacknowledge command from the interrogator in response to theinterrogator receiving a respective reply from a respective tag anddetermining the respective reply to be collision-free.
 39. The method ofclaim 38, further comprising each respective tag of the at least aportion of the population backscattering at least a portion of therespective identification code.
 40. The method of claim 39, furthercomprising the interrogator accessing a tag individually after receivingthe random number from the tag, accessing the tag including theinterrogator sending the random number to the tag.
 41. The method ofclaim 40, wherein the memory range of the memory of the tag includes atleast a portion of the random number.
 42. The method of claim 38,wherein each respective random number generated by each respective tagis sixteen bits in length.
 43. A method comprising: affixing a radiofrequency identification (RFID) tag to an object, the tag including tagmemory; disposing the tag in a communication field of an interrogator;sending a first signal from the interrogator to the tag after disposingthe tag in the field and before the tag communicates to theinterrogator, the first signal including parameters that describe amemory range and a bit string; receiving the first signal at the tag,and in response thereto, comparing the bit string against the memoryrange of the tag memory to determine if the tag is selected, the memoryrange of the tag memory storing a plurality of bits; the tag picking arandom value and associating the random value with a slot in accordancewith an arbitration scheme for an inventory operation if the tag isdetermined to be selected; sending a second signal from the interrogatorto the tag; backscattering a random number generated by the tag from thetag to the interrogator in accordance with the slot in response toreceiving the second signal; and sending a acknowledge command from theinterrogator to the tag in response to the interrogator receiving therandom number.
 44. The method of claim 43, further comprisingbackscattering at least a portion of an identification code from the tagto the interrogator, wherein the identification code is stored in tagmemory and identifies the object.
 45. The method of claim 44, furthercomprising the interrogator individually accessing the tag after theinterrogator sends the acknowledge command and receives the at least aportion of the identification code, wherein individually accessing thetag includes the interrogator sending an access command to the tag, theaccess command including the random number.
 46. The method of claim 45,wherein the random number is sixteen bits long.
 47. The method of claim43, wherein the plurality of bits includes at least a portion of therandom number.
 48. A method comprising: disposing a radio frequencyidentification (RFID) tag in a communication field of an interrogator,the tag including tag memory; sending a select command from theinterrogator to the tag after disposing the tag in the field and beforethe tag communicates to the interrogator, the select command includingparameters that describe a memory range and a bit string; receiving theselect command at the tag, and in response thereto, comparing the bitstring against the memory range of the tag memory to determine if thetag is selected, the memory range of the tag memory storing at least twobits; and communicating a random number generated by the tag from thetag to the interrogator in accordance with an arbitration scheme if thetag is determined to be selected.
 49. The method of claim 48, whereinthe random number is stored in the tag memory.
 50. The method of claim48, wherein the at least two bits include at least a portion of therandom number.
 51. The method of claim 48, further comprisingcommunicating at least a portion of an identification code from the tagto the interrogator in accordance with the arbitration scheme, whereinthe identification code identifies an object to which the tag isaffixed.
 52. The method of claim 51, wherein the identification code isstored in the tag memory.
 53. The method of claim 48, wherein the randomnumber is sixteen bits long.
 54. The method of claim 48, furthercomprising the tag picking a random value and using the random value asa slot, the tag communicating the random number at a time associatedwith the slot in accordance with the arbitration scheme.
 55. The methodof claim 54, further comprising sending an acknowledge command from theinterrogator to the tag in response to the interrogator receiving therandom number.
 56. The method of claim 55, further comprising sending asignal from the interrogator to the tag, after sending the selectcommand from the interrogator to the tag and before communicating therandom number from the tag to the interrogator, wherein the signalindicates to the tag the time to communicate the random number.
 57. Themethod of claim 48, further comprising sending a signal from theinterrogator to the tag, after sending the select command from theinterrogator to the tag and before communicating the random number fromthe tag to the interrogator, wherein the signal indicates to the tagwhen to communicate the random number to the interrogator.
 58. Themethod of claim 48, wherein communicating the random number includesbackscattering the random number.
 59. The method of claim 48, furthercomprising sending an acknowledge command from the interrogator to thetag in response to the interrogator receiving the random number.
 60. Themethod of claim 59, further comprising communicating at least a portionof an identification code from the tag to the interrogator in accordancewith the arbitration scheme, wherein the identification code identifiesan object to which the tag is affixed.
 61. The method of claim 60,further comprising the interrogator individually accessing the tag afterreceiving the random number, wherein individually accessing the tagincludes the interrogator sending an access command to the tag, theaccess command including a sixteen bit random number.
 62. The method ofclaim 61, wherein the sixteen bit random number is the random numbergenerated by the tag and communicated from the tag to the interrogatorin accordance with the arbitration scheme.
 63. A method comprising:disposing a plurality of radio frequency (RFID) tags in a communicationfield of an interrogator; sending a first signal from the interrogatorto first and second tags of the plurality of tags after disposing theplurality of tags in the field and before any of the plurality of tagscommunicate to the interrogator, the first signal including a bit stringand indicating a portion of memory, the portion of memory comprisingmultiple bit storage locations, the first tag having stored therein afirst set of bits in bit storage locations corresponding to the portionof memory, and the second tag having stored therein a second set of bitsin bit storage locations corresponding to the portion of memory; thefirst tag receiving the first signal and comparing the bit stringagainst the first set of bits to determine if the first tag is selected;the second tag receiving the first signal and comparing the bit stringagainst the second set of bits to determine if the second tag isselected; the first tag picking a first random value and associating thefirst random value with a first slot in accordance with an arbitrationscheme; the second tag picking a second random value and associating thesecond random value with a second slot in accordance with thearbitration scheme; the first tag backscattering a first identificationcode that identifies a first object to which the first tag is affixed;and the second tag backscattering a second identification code thatidentifies a second object to which the second tag is affixed.
 64. Themethod of claim 63, further comprising: the first tag backscattering afirst random number generated by the first tag; and the second tagbackscattering a second random number generated by the second tag. 65.The method of claim 64, further comprising: the interrogator receivingthe first random number from the first tag during a period of timeassociated with the first slot, and, in response thereto, theinterrogator sending a first acknowledge signal to acknowledge the firsttag; and the interrogator receiving the second random number from thesecond tag during a period of time associated with the second slot, and,in response thereto, the interrogator sending a second acknowledgesignal to acknowledge the second tag.
 66. The method of claim 65,further comprising the interrogator accessing the first tag individuallyafter receiving both the first random number and the firstidentification code from the first tag, accessing the first tagincluding the interrogator sending a command that includes a numberrandomly generated by the first tag that identifies the first tag. 67.The method of claim 66, wherein the number randomly generated by thefirst tag that identifies the first tag is the first random number, andthe first random number is 16 bits in length.
 68. The method of claim67, further comprising sending a second signal from the interrogatorafter sending the first signal from the interrogator, the first tagbackscattering the first identification code in response to receivingthe second signal.
 69. The method of claim 63, further comprising: theinterrogator sending a first acknowledge signal to acknowledge the firsttag; and the interrogator sending a second acknowledge signal toacknowledge the second tag.
 70. A method comprising: disposing a radiofrequency identification (RFID) tag in a communication field of aninterrogator; sending a first command from the interrogator to the tagafter disposing the tag in the field and before the tag communicates tothe interrogator, the first command including a first set of fieldscomprising at least two first bit values; the tag wirelessly receivingthe first command; the tag backscattering a first reply based, at leastin part, on whether the two first bit values received from theinterrogator match two corresponding bit values stored in the tag, thefirst reply including a random number generated by the tag; sending asecond command from the interrogator to the tag, the second commandincluding a second set of fields comprising at least two second bitvalues; the tag wirelessly receiving the second command; and the tagbackscattering a second reply based, at least in part, on whether thetwo second bit values received from the interrogator match the twocorresponding bit values stored in the tag, the second reply including arandom number generated by the tag.
 71. The method of claim 70, furthercomprising backscattering at least a portion of an identification codefrom the tag to the interrogator, wherein the identification codeidentifies an object to which the tag is affixed.
 72. The method ofclaim 70, further comprising the tag picking a random value and usingthe random value as a slot in accordance with an arbitration scheme, thetag backscattering a signal to the interrogator at a time associatedwith the slot.
 73. The method of claim 72, further comprising sending anacknowledge command from the interrogator to the tag.
 74. The method ofclaim 70, further comprising the interrogator individually accessing thetag, wherein individually accessing the tag includes the interrogatorsending an access command to the tag, the access command including asixteen bit random number.
 75. The method of claim 74, furthercomprising backscattering at least a portion of an identification codefrom the tag to the interrogator, wherein the identification codeidentifies an object to which the tag is affixed.
 76. The method ofclaim 70, further comprising the interrogator detecting a collision uponreceiving the first reply.